Green Synthesis Of Porous SiO₂/FeO_x Composite Fenton-like Catalysts And Their Degradation Of Dye-containing Wastewater

The treatment of dye-containing wastewater is a challenging problem in water pollution treatment.As a strong oxidation technology,Fenton catalysis has the characteristics of simple operation and mild treatment conditions,which is suitable for the treatment of dye-containing wastewater.However,the homogeneous Fenton catalysts are sensitive to p H,and the catalysts will produce iron sludge after the use,making them difficult to be recovered and recycled.Porous SiO₂ and iron oxide composite can be used as Fenton-like heterogeneous catalyst,which is expected to be used under a wide range of p H conditions and to eliminate the iron sludge.How to reduce or eliminate wastewater emission in the preparation of the composite catalysts is the key to their practical application.In this work,SiO₂ wet gel was prepared by sol-gel method using tetraethyl orthosilicate as silicon source,ethanol as cosolvent and low boiling point n-butyl amine as catalyst.All the reactants were included in the as-synthesized wet gel.Thus the gel was formed without wastewater formation.A set of self-made device was used for distillation and drying of the wet gel and the solvent and catalyst contained in the wet gel were recovered.More than 96%of the solvent and catalyst were recovered.The analysis of the recovered liquids indicated that the content of n-butylamine decreased gradually,but the content of ethanol increased.The recovered liquid was directly applied to prepare SiO₂ wet gels after adding TEOS for three times.The fourth recovery liquid could be only reused after its component adjustment.In the experiment,SiO₂ xerogels were prepared for 3 runs consisting of 12 cycles,and the xerogel synthesis was achieved without wastewater emission.The prepared SiO₂ xerogels were characterized in detail.The results showed that all the xerogels were composed of particles,forming a porous structure with high specific surface area.The surface area of SiO₂ xerogels synthesized by recovered liquid increased.The degree of linkage of[SiO₄]in the SiO₂xerogels increased after adjusting the reactants.The procedures of producing the SiO₂ xerogels were further applied to the preparation of SiO₂/FeO_x composite gel.The composite gel with a molar ratio of Fe/Si to 0.02:1?0.15:1 was synthesized by adding acetylacetone iron in the synthesis medium.The recovered liquid was reused to synthesize the composite gel for two times,and the resultant composite gels showed similar changes as in the SiO₂ xerogel.The results showed that the specific surface area of SiO₂/FeO_x composite gel was larger than that of SiO₂ xerogel,the pore size distribution wider and the macropores decreased.Iron was introduced and highly dispersed in the SiO₂ network of the composite gel via the formation of Si-O-Fe bonds.XPS analysis revealed that Fe²⁺/Fe³⁺co-existed in the composite gel,in which Fe²⁺and Fe³⁺accounted for 20%and 80%,respectively.The prepared composite gel was used for the adsorption of Rh6G.The introduction of iron species improved the removal efficiency of the gel toward Rh6G.The adsorption of Rh6G by the SiO₂/FeO_x composite xerogels followed the Freundlich and the pseudo-two order adsorption kinetic models,respectively.Thus the adsorption belonged to chemical adsorption and multi molecular adsorption.The whole adsorption process was controlled by external diffusion.Thermodynamic analysis showed that the adsorption was spontaneous and exothermic.The composite gel displayed no obvious degradation effect on Rh6G.The process and mechanism of adsorption and degradation of Rh B dye in wastewater using SiO₂/FeO_x composite gel as Fenton like catalyst were studied.The experimental results showed that the removal of dye Rh B increased from 83.4%to 99.8%after the addition of H₂O₂,indicating that the decolorization of dye-containing wastewater was due to the synergistic effect of adsorption and dye degradation.Similar results were obtained when the composite gel was prepared with the recovered liquid.The composite gel degraded Rh B under a wide p H range(3-11)condition.The composite gel was reused for 5 times,and the removal of Rh B was more than 99%.The SiO₂/FeO_x gel displayed high stability and the iron leaching were less than 0.35mg/L after each test,which met the requirement of national wastewater discharge standard and did not cause second pollution to the environment.The adsorption of dyes on the composite gel followed the pseudo-two order kinetic model,and the degradation process of Rh B followed the first-order reaction kinetics.Tiller modulus?in the process of adsorption and degradation was less than 0.5,indicating that the whole process was controlled by the catalytic degradation.In order to enhance the Fenton-like catalytic reaction,copper acetate was added in the synthesis medium for the porous SiO₂/FeO_x composite gel.The SiO₂/FeO_x composite gel containing CuO_x still exhibited porous structure,but the specific surface area decreased.The removal efficiency of Rh B and TOC in SiO₂/FeO_x composite gel containing CuO_x increased significantly in a wide p H range of 3-11,which was mainly due to fact that the Cu²⁺/Cu⁺electron pairs promoted the transformation of Fe³⁺to Fe²⁺.